Adjoint filter
# Adjoint filter

In signal processing, the adjoint filter mask $h^{*}$ of a filter mask $h$ is reversed in time and the elements are complex conjugated.

    $(h^{*})_{k}={\overline {h_{-k}}}$

Its name is derived from the fact that the convolution with the adjoint filter is the adjoint operator of the original filter, with respect to the Hilbert space $\ell _{2}$ of the sequences in which the inner product is the Euclidean norm.

    $\langle h*x,y\rangle =\langle x,h^{*}*y\rangle$

The autocorrelation of a signal $x$ can be written as $x^{*}*x$.

## Properties

* ${h^{*}}^{*}=h$
* $(h*g)^{*}=h^{*}*g^{*}$
* $(h\leftarrow k)^{*}=h^{*}\rightarrow k$
